A rocky and
water-rich planet, not much heftier than our own, has been discovered so close
to our solar system that astronomers one day may be able to study its
atmosphere.

And though
astronomers are pretty certain the water exists, they don't know its state,
with speculations ranging from liquid water to water ice and an exotic state
called a superfluid.

The
extrasolar planet, now named GJ 1214b, is about 40 light-years away. It orbits
a red dwarf star. It is the only known "Super-Earth" exoplanet ?
worlds that have masses between Earth and Neptune ? with a confirmed
atmosphere.

"Astronomically
speaking, this [planet] is on our block," meaning it's in our cosmic
neighborhood, said study leader David Charbonneau of the Harvard-Smithsonian
Center for Astrophysics in Mass. "For perspective, our own TV signals have
already passed beyond the distance of this star."

The planet
is about three times the size of Earth and about 6.5 times as massive. It is
the second smallest planet discovered outside of our solar system to date,
trailing behind only CoRoT-7b, which is 1.7 times Earth's size and about five times as massive.

GJ 1214b is
rare among known rocky
exoplanets because it partially eclipses, or transits, its star as seen
from Earth.

This
fortunate alignment allows astronomers to calculate the size and density of the
planet, and Charbonneau's team thinks GJ 1214b is likely a water world with a
solid center. Moreover, the planet has a thick surrounding atmosphere of
hydrogen and helium.

Normally, a
planet located at that distance from this particular type of star would be so
hot that any water on its surface would be in a vapor form.

But
scientists think the thick atmosphere of GJ 1214b creates a high pressure
environment that keeps water on the surface in a liquid state.

That's just
speculation, however.

"It
really depends on how hot the planet is on the inside, and we don't know that,"
Sara Seager, an astrophysicist at MIT, told SPACE.com. "I think this
planet doesn?t have liquid water because it is too hot on the inside. I think it
goes from water ice, to a very exotic kind of water ? a superfluid ? and then
it goes to vapor," added Seager, who just published a new eBook on
exoplanets called "Is
there Life Out There? The Search for Habitable Exoplanets."

(Seager was not involved in the new planet
discovery.)

There are
downsides to having such a thick atmosphere: First, the pressure is crushing,
making life as we know it difficult. And secondly, the thick atmosphere blocks
light from the feeble star from reaching the planet's surface.

"If
you picture the sun as a 1,000-watt light bulb, this star is a 3-watt light
bulb," Charbonneau said.

Whatever
its exact composition, astronomers are excited about the finding. "We're really looking for a planet that's
a big Earth orbiting in the Goldilocks
zone of a small star, transiting," Seager said. "This one scores
three out of four. We're excited because we're getting closer and closer to the
thing we want to find." The missing piece: GJ 1214b doesn't orbit within the star's habitable, or Goldilocks,
zone.

The planet
was discovered using a suite of small, ground-based telescopes and is detailed
in this week's issue of the journal Nature.